This invention relates to a multi-chip or stacked integrated circuit (IC) die device. More specifically, this invention is directed toward a multi-chip device having a plurality of IC die, for example, a plurality of memory die, a controller die and memory die, or a processor, controller and plurality of memory die.
One conventional technique employed to provide greater IC densification includes incorporating several IC die into a single package. There is demand for larger IC densification to more fully utilize system layout space for applications such as portable computers and cell phones. An attractive solution is offered by vertical integration of IC die. That is, a three-dimensional approach where IC die are disposed one on top of another to more fully utilize a vertical dimension.
With reference to FIG. 1, a conventional multi-chip device 100 having a plurality of IC die 110 and 120 disposed one on top of another is illustrated. IC die 110 is stacked on top of IC die 120 which is disposed on base or substrate 130. A plurality of bond wires 140a-140d electrically couple pads disposed on IC die 110 to pads disposed on IC die 120. Similarly, a plurality of bond wires 150a-15m electrically couple pads disposed on IC die 120 to pads disposed on base 130.
The physical configuration of multi-chip device 100 tends to relax demands on system layout space. By disposing the IC die (e.g., memory die) vertically, only a single IC die footprint is required thereby resulting in a reduction in system layout space from a lateral or horizontal perspective Signal lines 140a-140d for example, control lines and address/clock lines are routed vertically between the IC die 110 and 120.
Contemporary stacked die configurations tend to minimize the length of the signal line path. That is, the contemporary approach for stacked die configurations, is to minimize the electrical path length of the signal lines. A stacked die configuration which seeks to minimize the length of the signal lines is described and illustrated in U.S. Pat. Nos. 5,675,180 and 5,698,895.
Contemporary stacked die configurations, like those described and illustrated in U.S. Pat. Nos. 5,675,180 and 5,698,895, employ a minimum signal line path length in order to decrease propagation delay of the signals on those lines. Decreasing the propagation delay tends to decrease the travel time of the signal between the die and, in turn, increase the speed of operation of the overall system.
In addition to presenting a minimum propagation delay of the signals applied to the signal lines, stacked die configurations employing a minimum signal line path length tend to minimize parasitic capacitance and inductance resulting from the interconnects. In general, this approach may promote faster operation because signal line lengths and corresponding propagation delays are reduced.
FIG. 2A is a schematic diagram of a plurality of conventional IC die in a stacked die configuration coupled to a signal line employing minimal conductor length between each IC die. Here the plurality of IC die 310a-310d are inter-coupled via conductors 320a-320d, respectively. Conductors 320a-320d represent a signal line. Load capacitances 330a-330d represent the load capacitance presented by IC die 310a-310d which are coupled to the signal line. In this regard, when electrically coupling IC die to signal bussing, the signal lines become loaded with the inherent load capacitance which is due to the various elements of the I/O structures disposed on the integrated circuit, for example, bond pads, electrostatic discharge protection devices, input buffer transistor capacitance, and output driver transistor parasitic and interconnect capacitances relative to the memory device substrate.
Because the length of conductors 320a-320d are minimized, conductors 320a-320d exhibit, as a practical matter, negligible inductance. Thus, load capacitances 330a-330d are effectively lumped producing a large overall equivalent or lumped capacitive characteristic. Here, the capacitive characteristic is present between a ground plane 340 which is common to IC die 310a-310d and conductors 320a-320d.
One method for providing an increase in bandwidth and overall performance of a memory system, is to increase the effective data rate at which data may be transferred to and from each memory device (i.e., the data rate). In memory systems, one conventional approach to achieve such an increase is to increase the clock rate of the system, which tends to increase the data rate of the system and, in turn, the bandwidth.
However, as the data rate increases in multi-chip devices which seek to minimize signal line lengths, the lumped capacitive characteristic mentioned above requires increasingly more drive capability from output drivers of the IC die 310a-310d attempting to drive data onto conductors 320a-320c at an increased rate. That is, as the data rate increases, a relatively large amount of current is necessary in the same given period of time to drive the large overall lumped capacitive characteristic at a faster rate. Driver current for an output driver transmitting on an un-terminated lumped capacitive load is illustrated in FIG. 2B. The current demand from drivers of the signal line correspondingly increases with an increase in data rate for a given lumped load capacitance.
As data rates increase in systems employing minimal or short signal lines between IC die of a stacked die device, the number of IC die which may be coupled along the signal line decrease. As mentioned above, minimal or short signal lines between IC die of a stacked die device tend to result in negligible inductance separating each load capacitance along the signal line. Since each IC die increases the overall lumped load capacitance of the signal line in such a system, the maximum practical number of IC devices which may be coupled to the same signal line tends to become constrained or limited by the drive capability of the drivers on the IC die.
Stacked die configurations employing a minimal conductor length provide relatively fast access times. These configurations, however, suffer a number of shortcomings including a limitation on the maximum practical number of IC devices which may be coupled to the same signal line—i.e., a limit on the amount of vertical integration. That is, the minimum interconnect stacked die configurations place high demands on the necessary output drive which imposes an operation speed limitation on the system or a limitation on the number of devices or die coupled to the signal line. Thus, there is a need to provide an effective configuration which has fast access times, increases the operation speed of a multi-chip or stacked die device, and provides more flexibility in vertical integration.